This presentation intends to offer a bird's eye view of Carbon assimilation in plants along with diversity.

1. PLANT METABOLISM
Carbon Assimilation : Biology & Chemistry along with
diversity
By
N. Sannigrahi, Associate Professor,
Department Of Botany,
Nistarini College, Purulia (W.B) INDIA

2. WHAT IS CARBON ASSIMILATION?
 Light dependent reactions assures the production of
assimilatory powers and reducing powers required for the
reduction of atmospheric carbon into carbohydrate molecules
and the path of carbon in photosynthesis includes those
reactions which incorporate carbon into more reduced or more
energetic compounds by a number of enzyme mediated
biochemical reactions. There are three major and one minor
pathways by which atmospheric CO2 can be assimilated in
photosynthesis. The first is the Calvin cycle or C3 cycle since
the early product in this pathway is a C3 compound 3-
Phosphoglyceric acid (PGA). It is also known as reductive
pentose phosphate pathway or photosynthetic carbon reduction
cycle (PCR). The second is called C4 cycle because the early
products are C4 acids- malate or aspartate , while terminal
steps include the reactions of Calvin cycle.

3. CARBON ASSIMILATION
 In the third group, much of the CO2 is fixed in the process
known as Crassulacean Acid Metabolism (CAM) , a
specialized pattern of photosynthesis in which CO2 is
absorbed and stored at night as malic acid and released during
the day by decarboxylation inside the tissue in which it is fixed
by the Calvin cycle. This permits the economic use of the
water as an exercise of the water conservation because the
stomata can remain closed during the day time when there is
almost no CO2 assimilation directly from the air. Intimately
connected with dependent on the C3 cycle, there is another
minor cycle named as C2 photo-respiratory carbon oxidation
cycle or the C2 cycle. Another pathway is exhibited by aquatic
alga which posses mechanisms by actively acquiring inorganic
carbon (c1) such as CO2 and HCO3- from the external
medium and are able to use this C1 to elevate the CO2
concentration around the active site of RUBISCO under
extremes CO2 limiting condition.

4. CARBON ASSIMILATION-DIVERSITY
 These plants may be called C-1 plants and the pathway
adopted by them can be considered as C1 pathway. Just like
C4 plants where 4-C compound donates CO2 by de-
carboxylation , 1-C compound like HCO3- in these C-1 plants
donates CO2 for photosynthetic fixation.
 From the above different biochemical variants of
photosynthetic assimilation of CO2, it is possible to make two
important general conclusions.
 1. The C3 cycle is the only known sequence of reactions
capable of photosynthetic conversion of CO2 carbohydrate.
One of the important characteristics of the C3 cycle is its auto
catalytic ability of generating CO2 acceptor molecule RuBP,
while the other cycles do not have this property. In fact, the
other pathways require operation of several enzymes in
addition to those Calvin cycle, whereas the reactions of Calvin
cycle are common to all plants.

5. CARBON ASSIMILATION-DIVERSITY
 The Calvin cycle regenerates its own biochemical components
that are necessary to maintain the operation of the cycle. The
rate of the operation of the cycle can be enhanced by
increasing the concentration of the interemediates.If the leaves
or isolated chloroplasts are kept in dark are illuminated CO2
fixation starts only after a lag period , called induction period
and the rate of photosynthesis increases with time. This
increase is partly due to the activation of enzymes by light and
partly due to the concentration of intermediates.
 2 .Other pathways like the C4 and CAM provide a mechanism
in plants for concentrating CO2 at the site of RuBP
carboxylation and thus improve the CO2- absorbing capacity
of the plants. The C2 cycle is an inevitable consequences of
the C3 cycle reaction and both cycles are integrated together.

6. CARBON ASSIMILATION
 The Calvin cycle (C3-cycle) or PCR-cycle can be divided
into three stages:
 (a) Car-boxylation, during which atmospheric CO2 combines
with 5-C acceptor molecule ribulose 1, 5-bisphosphate (RuBP)
and converts it into 3-phosphoglyceric acid (3-PGA);
 (b) Reduction, which consumes ATP + NADPH (produced
during primary photochemical reaction) and converts 3-PGA
into 3-phosphoglyceraldehyde (3PGAld) or triose phosphate
(TRI- OSE-P); and
 (c) Formation of hexose sugar and regeneration of RuBP
which consumes additional ATP, so that the cycle continues .
 The all the steps of the carbon assimilation arte catalyzed by
the different enzymes along with the reducing powers and
assimilatory powers harvested from the light dependent
biochemical pathways take place in the grana regions of
mesophyll tissues of the green plants.

7. CALVIN CYCLE

8. STEPS OF CARBON ASSIMILATION
 Calvin cycle was outlined by Calvin and his Co-researchers
(Benson, et al., 1950, Bassham & Calvin, 1957; Calvin &
Bassham, 1962) This cycle has been found in all
photosynthetic organisms so far and the details are as follow.
 Carboxylation :(i) The CO2 is accepted by Ribulose 1, 5-
bisphosphate (RuBP) already present in the cells and a 6-
carbon addition compound is formed which is unstable. It soon
gets hydrolyzed into 2 molecules of 3-phosphoglyceric acid
(3PGA). Both these reactions take place in the presence of
ribulose bisphosphate carboxylase (Rubisco). 3-
Phosphoglyceric acid is the first stable product of dark reaction
of photosynthesis.
 (b) Reduction: (ii) 3-Phosphoglyceric acid is reduced to 3-
phosphoglyceraldehyde by the assimilatory power (generated
in light reaction) in the presence of triose phosphate
dehydrogenase.

9. DARK REACTION
 c) Formation of Hexose Sugar and Regeneration of RuBP:
 (iii) Some of the molecules of 3-phosphoglyceraldehyde
isomerizes into dihydroxyaeetone phosphate, both of which
then unite in the presence of the enzyme aldolase to form fruc-
tose 1, 6-bisphophate.
 iv) Fructose 1, 6-bisphosphate is converted into fructose 6-
phosphate in the presence of phosphatase.
 (v) Some of the fructose-6-phosphate (hexose sugar) is tapped
off from the Calvin cycle and is converted into glucose,
sucrose, and starch. Sucrose is synthesized in cytosol while
starch is synthesized in chloroplast.
 (vi) Some of the molecules of 3-phosphoglyceraldehyde
produced in step (ii) instead of forming hexose sugars, are
diverted to regenerate ribulose 1, 5-bisphosphate in the system
as follows:

10. CALVIN CYCLE

11. CALVIN CYCLE
 vii) 3-Phosphoglyceraldehyde reacts with fructose-6-
phosphate in the presence of enzyme transketolase to form
erythrose-4-phosphate (4-C atoms sugar) and xylulose 5-
phosphate (5-C atoms sugar).
 (viii)Erythrose-4-phosphate combines with dihydroxyaceotone
phosphate in the presence of the enzyme aldolase to form
sedoheptulose 1, 7-bisphosphate (7-C atoms sugar).
 ix) Sedoheptulose 1, 7-bisphosphate loses one phosphate
group in the presence of phosphatase to form sedoheptulose-7-
phosphate.
 (x)Sedoheptulose-7 phosphate reacts with 3-
phosphoglyceraldehyde in the presence of transketolase to
form xylulose-5-phosphate and ribose-5-phosphate (both 5-
carbon atoms sugars).

12. CALVIN CYCLE
 (xi) Xylulose-5-phosphate is converted into another 5-C
atoms sugar ribulose-5-phosphate in the presence of the
enzyme phosphoketopentose epimerase.
 (xii) Ribose-5-phosphate is also converted into ribulose-5-
phosphate. The reaction is catalyzed by phosphopentose
isomerase.
 (xiii) Ribulose-5-phosphate is finally converted into
ribulose 1, 5-bisphosphate in the presence of
phosphopentose kinase and ATP, thus completing the Calvin
cycle.
 Because first visible product of this cycle is 3-
phosphoglyceric acid which is a 3-C compound, Calvin
cycle is also known as C3-pathway. (Recent studies with
algal cells, leaves and isolated chloroplasts have shown that
‘dark reactions’ of photosynthesis are not completely
independent of light

13. NET REACTION OF CALVIN CYCLE

14. C4- PHOTOSYNTHETIC CYCLE
 A major new pathway for carbon flow during photosynthesis
other than Calvin cycle was established in sugarcane, maize,
sorghum and related grasses. Initial studies by Kaprilov (1960)
in Russia on maize and by Kortschak, Hart & Burr (1965) in
Hawaii on sugarcane revealed that the major early products by
photosynthesizing leaves exposed to Radioactive carbon
sources and C4 acids malate and aspartate. Hatch & slack
(1966) in Australia confirmed the earlier observation and
proposed a cyclic reaction mechanism in which a C3 acid is
carboxylated to yield a C4 acid and subsequently donates one
carbon as CO2 to the reductive photosynthetic cycle or Calvin
cycle where the second carboxylation takes place. The new
pathway was originally named as C4 dicarboxylic acid
pathway . It is now referred as C4 pathway and the plants
exercised are called C4 plants.

15. HATCH-SLACK CYCLE
 Thus, C4 plants can be defined on the basis of the following
characters:
 Primary initial products of CO2 fixation are the 4-carbon
dicarboxylic acids OAA , malate and aspartate. Hence, the
name has been derived from initial carbon fixation.
 CO2 fixation into C4 acids occurs in the light not in darkness
like CAM plants.
 C₄ carbon fixation or the Hatch–Slack pathway is one of three
known photosynthetic processes of carbon fixation in plants. It
owes the names to the discovery by Marshall Davidson Hatch
and Charles Roger Slack that some plants, when supplied with
¹⁴CO 2, incorporate the ¹⁴C label into four-carbon molecules
first.

16. HATCH-SLACK CYCLE
 1.Hatch-Slack Cycle operates in C4 plants only.
 2. Hatch-Slack Cycle has a faster rate of CO2 fixation.
 3. Fixed CO2 is released back in bundle sheath cells where it is
finally fixed and reduced by Calvin cycle.
 4. The primary acceptor of CO2 is PEP, a 3-carbon compound.
 5. The first stable product is OAA, a 4-carbon compound.
 6. Hatch-Slack Cycle can operate under very low CO2
concentration.
 7. Fixation of one molecule of CO2 requires 2 ATP molecules
in addition to that required in C3 cycle.
 8. Optimum temperature for the operation of C4 cycle is 30 to
45°C.

17. HATCH-SLACK CYCLE
 9. Hatch-Slack Cycle has no such gain.
 10.C4 quantum yield independent of Co2 concentration and
temperature.
 11.C4 plants are most efficient and abundant in hot, dry and
high light habitats,
 12. All C4 plants are basically C3 as glucose synthesis is done
by Calvin cycle avenue.
 The C3 acid remaining after C4 acid de-carboxylation diffuses
back into the mesophyll cells where it is converted to PEP by
the enzyme pyruvate orthophosphate dikinase , thus
regenerating the CO2 acceptor. The last step is the critical
operation of the process.

18. DIMORHISM OF CHLOROPLAST

19. GENERA HAVING BOTH C3 & C4
Serial
No.
Family Name Genus Name
1. Aizoaceae Mollugo
2. Amaranthaceae Aerva, Alternanthera
3. Boraginaceae Heliotropium
4. Chenopodiaceae Artplex, Brassica, Kochla, Suaeda
5. Asteraceae Flaveria, Pectis
6. Cyperaceae Cyperus, Scripus
7. Euphorbiaceae Chamaesyce, Euphorbia
8. Poaceae Alloteropis, Panicum
9. Nyctaginaceae Boerhaavia
10. Zygophyllaceae Kallstroemia, Zygophyllum

20. THREE VARIANTS OF C4 PHOTOSYNTHESIS
Seria
l No.
Principal
C4 acids to
the BSC
Decarboxylating
enzymes
Variant
name
Principal C3
acids
returned to
MC
Examples
1. Malate NADP dependent
malic enzyme
(chloroplast)
NADP-ME Pyruvate Maize, Crab
grass,
Sorghum
2. Aspartate NAD dependent
malic enzyme(
Mitichondrial)
NAD-ME Alanine Millet
(Panicum
miliacium)
3. Aspertate PEPcase (Cytosol) PEP-CK Alanine/Pyr
uvate
Guinea
grass(
Panicum
maximum)

21. HATCH-SLACK CYCLE

22. REACTIONS OF C4 CYCLE
 1.Fixation of CO2 by the carboxylation of
phosphoenolpyruvate in mesophyll cells(MC) to form OAA or
Malic or Aspartic acid,
 2. Transport of the C4 acids into Bundle Sheath Cell(BSC)
 3. Decarboxylation of the C4 acids within the BSC and the
generation of CO2 which is then reduced to carbohydrate via
Calvin cycle,
 4. Transport of the C3 acids –pyruvate or alanine that is
formed by the decarboxylation step back to the mesophyll cells
and the regeneration of the CO2 acceptor of PEP.
 The undesired expenditure of 2 ATPs per CO2 fixation is an
unavoidable loss of this mechanisms and the usual NADHH is
used for the reduction of CO@ by the usual process of Calvin
cycle.

23. HATCH-SLACK CYCLE

24. MECHANISM
 However, in category of C4 plants, nature has evolved a
mechanism to avoid occurrence of photorespiration, which is
thought to be a harmful process.
 C4 pathway requires the presence of two types of
photosynthetic cells, i.e., mesophyll cells and bundle sheath
cells. The bundle sheath cells are arranged in a wreath like
manner. This kind of arrangement of cells is called Kranz
anatomy (Kranz: wreath). In Kranz anatomy, the mesophyll
and bundle sheath cells are connected by Plasmodesmata or
cytoplasm bridges.
 The C4 plants contain dimorphic chloroplasts. The chloroplasts
in mesophyll cells are granal, whereas in bundle sheath cells
they are agranal.

25. MECHANISM
 The granal chloroplasts contain thyllakoids which are stacked
to form grana, as formed in C3 plants. However, in agranal
chloroplasts of bundle sheath cells grana are absent and
thylakoids are present only as stroma lamellae.
 The presence of two types of cells (granal and agranal) allows
occurrence of light and carbon (dark) reactions separately in
each type.
 Here, release of O2 takes place in one type, while fixation of
CO2 catalyzed by Rubisco enzyme occurs in another type of
cells.
 In C4 plants (maize, sugarcane, etc.), light reactions occur in
mesophyll cells, whereas CO2 assimilation takes place in
bundle sheath cells. Such arrangement of cells does not allow
O2 released in mesophyll cells to enter in bundle-sheath cells.

26. OVERALL PATHWAY

27. UNIQUENESS
 Hence, Rubisco enzyme, which is present only in bundle-
sheath cells, does not come into contact with O2, and thus,
oxygenation of RuBP is completely avoided.
 In C4 plants, a CO2 concentrating mechanism is present which
helps in reducing the occurrence of photorespiration (i.e.,
oxygenation of initial acceptor RuBP). This type of CO2
concentrating mechanism is called C4 pathway.
 The members of the families having this photosynthetic
pathways are-
 Acanthaceae, Aizoaceae, Amaranthaceae, Boraginaceae,
Cappridaceae, Caryophyllaceae, Asteraceae, Cyperaceae,
Euphorbiaceae, Poaceae, Nyctaginaceae, Polygonaceae,
Portulacaceae, Scrophulariaceae, Zygophylaceae etc.

28. CRASSULACEAN ACID METABOLISM

29. CAM –WHAT IS & WHERE ?
 This type of metabolism, refers to a mechanism of
photosynthesis, that is, different from C3 and C4 pathways.
Crassulacean acid metabolism (CAM) is found only in
succulents and other xerophytes or plants that grow in dry
conditions. In this type of metabolism, CO2 is taken up by the
leaves on green stems through stomata which remain open
during night. However, during day time, stomata in such plants
remain closed to conserve moisture. The CO2 taken up by
succulent plants in night is fixed in the similar way as it takes
place in C4 plants to form malic acid, which is being stored in
vacuole. Hence, malic acid formed during night is used during
day time as a source of CO2 for photosynthesis to proceed
through C3 pathway. Crassulacean metabolism is a kind of
adaptation found in certain succulent plants such as pineapple
to proceed photosynthesis without much loss of water, which
generally occurs in plants with C3 and C4 pathways.

30. CAM CYCLES

31. SIGNIFICANCE OF CAM
 Significance of CAM Cycle. Crassulacean Acid Metabolism
or CAM cycle. It is one of the carbon pathways identified in
succulent plants growing in semi-arid or Xeric condition. This
was first observed in Crassulaceae family plants like
Bryophyllum, Sedum, Kalanchoe and is the reason behind
the name of this cycle.
 It is well adapted to hot, dry environments,
 Uptake of CO2 at night when CO2 is mostly readily available
in vernal plants,
 Trade off desiccation or starvation,
 CO2 acquisition at night provides competitive damage,
 High energy costs and low CO2 assimilation rates result in low
productivity.

32. MECHANISM OF CAM
 It is interesting to know that in the plants possessing Calvin
cycle, the enzyme RuBP carboxylase can initiate the reversal
of photosynthetic reactions. This process occurs when there is
low CO2, concentration but high O2, concentration. At mid-
day, when temperature and CO2 content are high, the affinity
of RuBP carboxylase increases for O2 but decreases for CO2.
Thus, it converts RuBP to 3-carbon compound (PGA) and a 2-
carbon compound (phosphoglycolate). The phosphoglycolate
is converted rapidly to glycolate in the peroxisomes.
 Glycolate is further converted to glycine, serine, CO2 and NH3
without the generation of ATP or NADPH. Thus net result is
oxidation of organic food synthesized during photosynthesis.
This process is called photorespiration or glycolate pathway as
it occurs at high rate in the presence of light. As already
mentioned that photorespiration is a loss to the net productivity
of green plants having Calvin cycle.

33. MECHANISM OF CAM
 The green plants having Calvin cycle are C3 plants.
Overcoming photo-respiratory loss poses a challenge to plants
growing in the tropics. Photorespiration occurs due to fact that
the active site of enzyme Rubisco (ribulose bisphosphate
carboxylase oxygenase) is same for both carboxylation and
oxygenation. The oxygenation of RuBP (ribulose
bisphosphate) in the presence of O2 is first reaction of
photorespiration that leads to the formation of one molecule of
phosphoglycolate, a two-carbon compound and one molecule
of PGA. Where PGA is used in Calvin cycle, and
phosphoglycolate is dephosphorylated to form glycolate in the
chloroplast.

34. C2 CYCLE

35. MECHANISM
 From chloroplast, glycolate is diffused to peroxisome where it
is oxidised to in glyoxylate. Here glyoxylate is used to form
amino acid, glycine. Now, glycine enters mitochondria where
two glycine molecules (4 carbons) give rise to one molecule of
serine (3 carbons) and one molecule of CO2 (one carbon).
Now, serene is taken up by peroxisome, and through a series
of reactions is being converted into glycerate.
 This glycerate leaves the peroxisome and enters the
chloroplast, where it is phosphorylated to form PGA. Now
PGA molecule enters the Calvin cycle to make carbohydrates,
but one CO2 molecule released in mitochondria during
photorespiration has to be re-fixed. This means, 75 per cent of
the carbon lost by the oxygenation of RuBP is recovered and
25 per cent is lost as release of one molecule of CO2.
 Photorespiration is also known as photosynthetic carbon
oxidation cycle.

36. SIGNIFICANCE
 Photorespiration is a respiratory process in many higher plants.
This is also known as the oxidative photosynthetic, or
C2 photosynthesis or carbon cycle. Sometimes in C3 plants,
RuBisCO binds to oxygen molecules and the reaction deviates
from the regular metabolic pathway. The combination of RuBP
and oxygen molecules leads to the formation of one molecule
of phosphoglycerate and phosphoglycolate. This pathway is
called photorespiration. During photorespiration, no sugar or
ATP molecules are synthesized, but just CO2 is released at the
expense of ATP and the whole process is futile.
 It seems probable that photorespiration serves to protect the
photochemical apparatus from light damage by the dissipation
of photochemical energy which concomitant CO2 assimilation
by consuming light generated reductant.

37. COMPARSION AMONG C3, C4 & CAM
C3 plants C4-Plants CAM-Plants
Plants operate Calvin cycle
only in all green cells
Plants operate C4 cycles in
MC in addition to C3 cycle
in BSC
Plants operate only C3
cycle in MC for carbon
assimilation.
Only CO2 acceptor is
RuBP
Two CO2 acceptors-PEP &
RuBP
Same as C4
The first stable product is
PGA (C3 acid).
The first stable product is
C4 compound
The initial fixation product
is C4 compound.
Kranz anatomy of
dimorphism of chloroplast
absent.
Dimorphism of
chloroplasts in the name of
Kranz anatomy is observed
No Kranz anatomy.
There is no concentrating
device , fixation and
assimilation of C takes
place through Calvin cycle
in the day. No
decarbxylation mechanism.
There is initial CO2
concentrating mechanisms
with the involvement of BC
& BSC
Night acidification
followed by light
decarboxylation is found
Photorespiration is
prominent.
Photorespiration can be
detected due to PEPcase
Photorespiration can not be
detected

38. C1- PHOTOSYNTHETIC CYCLE
 Another operation of a CO2 concentrating mechanism ( CCM)
has been discovered recently and is exhibited by aquatic algae
where the dissolved inorganic carbon (C1) is transported into
the cell across the plasma membrane in the form of either CO2
or HCO3(-) but stored there as HCO3 (-) ions. Its
decarboxylation i.e the formation of CO2 and H2O by the
enzyme carbonic anhydrase (CA) leads to enrichment with
CO2 around the active site of rubisco under the extreme CO2
limiting condition. This results in enhanced affinity for CO2
and improved photosynthetic efficiency.
 There are four major components of CO2 concentrating
mechanisms (CCM) in an organisms showing C1
photosynthesis as follows:
 1. Mechanism to transport inorganic carbon(C1) into cell and
chloroplast situated in the plasma membrane,

39. C1- PHOTOSYNTHETIC CYCLE
 2.An energy supply system to drive CO2 & HCO3(-) across
plasma membrane which is linked with plasma-membrane
bound ETS and ATPase driven proton pump,
 3.A CO2 leakage control device to reduce the efflux of CO2
out of the cell to the surrounding medium. Excess CO2 may be
maintained as HCO3(-) pool inside the cell. For this reason, a
micro environment developed in algae in the form of
carboxysomes in cyanobacteria and pyrenoids in chloroplast
of green algae. Both Rubisco and CA (carbonic dehydrase)
remain in these structures. This enzyme driven pathway is
associated with the carbon assimilation mechanisms.
 4.Submerged Aquatic Microphytes ( SAM) show mixed
pathways .The SAM plants exhibit all the known types of
CCM.

40. FACTORS AFFECTING CARBON REDUCTION
 “When a process is conditioned as to its rapidity by a number
of separate factors, the rate of the process is limited by the
pace of the slowest factor.”-this is known as Blackman's law of
limiting factors. In addition to these, the cardinal values also
play an important role for the regulation of photosynthesis
process. There are lot of factors having direct and indirect
impact upon the photosynthesis .These are classified as
Internal and External factors.
 INTERNAL: Chlorophyll, Protoplasmic factor, Leaf anatomy,
Ultra structure of Chloroplast
 EXTERNAL: Sunlight( Quality, Intensity, Duration),
Temperature, Water, Oxygen .

41. CARDINAL VALUES
 Theory of three cardinal points was given by Sachs in 1860.
According to this concept, there is minimum, optimum and
maximum for each factor. For every factor, there is a minimum
value when this mechanisms of photosynthesis starts, an
optimum value showing highest rate and a maximum value,
above the limit the reactions become fails to take place.
 Law of Limiting Factor:
 The most advocated theory that states the regulation of
biochemical reactions in presence of number of variables is
the law of limiting factors given by Blackman in 1905. When
several factors affect any biochemical process, then this law
comes into effect. This states that: if a chemical process is
affected by more than one factor, then its rate will be
determined by the factor which is nearest to its minimal value.

42. FACTORS OF PHOTOSYNTHESIS
 When CO2, light and other factors are not limiting, the rate of
photosynthesis increases with a rise in temperature, over a
range from 6°C to about 37°C. Above this temperature, there
is an abrupt fall in the rate and the tissue dies at 43°C. High
temperatures cause the inactivation of enzymes and therefore
affect the enzymatically controlled ‘dark’ reactions of
photosynthesis.
 The optimum temperature for the maximum falls between 20-
30°C. Above 25-30°C the maximum rate is not maintained as
the time factor begins to operate and the optimum temperature
is reduced from 37°C to 30°C. Given other factors are limiting,
the rate of photosynthesis follows Vant Hoffs rule between
6°C-30°C to 35°C i.e., it doubles with each increase of 10°C.
The reason being that all the reactions of the Calvin cycle are
temperature dependent and the rate of diffusion of CO2 to the
chloroplasts is accelerated by high temperature.

43. CARBON-DI-OXIDE
 Nearly 0.032% by volume of carbon dioxide is present in the
atmosphere and at this low level it acts as a limiting factor.
Under laboratory conditions when light and temperature are
not limiting factors, increase in CO2 concentration in the
atmosphere from 0.03% to 0.3-1% raises rate of
photosynthesis.
 With the further increase in the concentration of CO2
progressively the rate of carbon assimilation increases slightly
and then it becomes independent of CO2 concentration.
 Thereafter, it remains constant over a wide range of CO2
concentrations. Plants vary in their ability to utilize high
concentrations of CO2. In tomatoes, high concentration of
CO2, above the physiological range, exerts harmful influence
causing leaf senescence. During the early period of the earth,
the concentration of CO2 in the atmosphere was as high as
20%.

44. LIGHT
 The photo synthetically active region of the spectrum of light
is at wavelengths from 400-700 nm. Green light (550 nm)
plays an important role in photosynthesis. Light supplies
energy for the process.
 Light varies in intensity, quality and duration. A brief account
on these three aspects is given as follows:
 When CO2 and temperature are not limiting and light
intensities are low, the rate of photosynthesis increases with an
increase in its intensity. At a point saturation may be reached,
when further increase in light intensity fails to induce increase
in photosynthesis. Optimum or saturation intensities may vary
with different plant species e.g., C4 and C3. C3 plants become
saturated at levels considerably lower than full sunlight but C4
plants are usually not saturated at full sunlight.

45. OXYGEN
 Oxygen has been shown to inhibit photosynthesis in C3 plants
while C4 plants show little effect. It is suggested that C4 plants
have photorespiration and high O2 stimulates it. The rate of
photosynthesis increases by 30-50% when the concentration of
oxygen in air is reduced from 20% to 0.5% and CO2, light and
temperature are not the limiting factors.
 Oxygen is inhibitory to photosynthesis because it would favor
a more rapid respiratory rate utilizing common intermediates,
thus reducing photosynthesis. Secondly, oxygen may compete
with CO2 and hydrogen becomes reduced in place of CO2.
Thirdly, O2 destroys the excited (triplet) state of chlorophyll
and thus inhibits photosynthesis.
 It may be stated that direct effect of O2 on photosynthesis
remains to be understood.
 The different factors play a very crucial role in this regard.

46. WATER
 Water is an essential raw material in carbon assimilation. Less
than 1% of the water absorbed by a plant is used in
photosynthesis. The decrease in water contents of the soil from
field capacity to the permanent wilting point results in the
decreased photosynthesis.
 The inhibitory effect is primarily attributed to increased
dehydration of protoplasm and also stomatal closure. The
removal of water from the protoplasm also affects its colloidal
state, impairs enzymatic efficiency, inhibits vital processes like
respiration, photosynthesis etc. Dehydration may even damage
the micro molecular structure of the chloroplasts.
 It is also assumed that primary factor of dehydration in
retarding photosynthesis is due to stomatal closure which
reduces CO2 absorption. Water deficiency may cause drying of
the cell walls of mesophyll cells, reducing their permeability to
CO2. Water deficiency may accumulate sugars and thus
increase respiration and decrease photosynthesis.

47. CHLOROPHYLL & CHEMICALS
 The rate of photosynthesis in two varieties of barley having
normal green leaves and yellow leaves was studied. CO2, light
and temperature were not limiting factors. The rate of
assimilation per unit area of leaf surface in the two varieties
was the same even though the green-leaved variety contained
ten times more chlorophyll than the yellow one. Clearly, the
chlorophyll in the green leaves is surplus. Leaves having high
chlorophyll content do not photosynthesize rapidly since they
lack the enzymes or co-enzymes to use the products of the
light reactions to reduce available CO2.
 Compounds like HCN, H2S, etc. when present even in small
quantities, depress the rate of photosynthesis by inhibiting
enzymes. In addition chloroform, ether etc., also stop
photosynthesis and the effect is reversible at low
concentrations. However, at high concentrations the cells die

48. CONCLUSION
 Thus, photosynthesis is the unique property of plants involving
the energy transduction by the light energy converted into
chemical energy necessary for the different vital functions of
the organisms. The starch produced by the plants converted
into different other biochemical ingredients as reflected by the
yield of the different type of the plants. The biology and
chemistry of photosynthesis and diversity of this unique
mechanisms are great concern of the scientists. The C3. C4,
C2 and C1 along with CAM are the different diverse modes of
carbon assimilation of plants corresponding to the different
ecological conditions as avenues for adaptations. The
consumers depend upon autotrophy and the human beings
drink a glass of sunlight indirectly as means of energy and
transduction. Still, photosynthesis is the magic of reality to the
plant biologists,

49. THANKS FOR YOUR JOURNEY
 Acknowledgement:
 1. Google for images
 2. Different web pages for content and enrichment,
 3.Plant Physiology- Mukherji & Ghosh
 Applied Plant Physiology- Arup Kumar Mitra
 A text book of Botany- Hait, Bhattacharya & Ghosh
 Plant Physiology-Devlin
 Disclaimer: This presentation has been prepared for online
free study materials for academic domain without any
financial interest.